Lithium ion secondary batteries have two advantageous properties of high energy density and high power density and hence are at a higher level of perfection, and widely used as power sources for mobile devices. However, in recent years, the mobile devices have higher performance and are likely to have increased power consumption, and therefore a lithium ion secondary battery having further improved energy density and power density is demanded.
For meeting the demands, studies are made on the improvement of the lithium ion secondary battery in energy density, for example, a change of the electrode material for positive electrode/negative electrode, an improvement of the method of applying the electrode material, and an improvement of the method of sealing the electrode material are made, but the results cannot be put into practical use. A remarkable improvement in the energy density cannot be expected unless otherwise the materials currently used in the lithium ion secondary battery are changed.
For this reason, the development of a battery having an energy density higher than that of the lithium ion secondary battery is earnestly desired, and a fuel cell is one of the promising candidates.
The fuel cell includes an anode electrode and a cathode electrode, and a fuel is supplied to the anode electrode side, and air or oxygen is fed to the cathode electrode side. The fuel is supplied to cause a redox reaction, and electric energy can be taken from the chemical energy of the fuel.
A number of types of fuel cells are present in the world, and, according to the electrolyte membranes used in the fuel cells, they are classified into an alkali system (AFC), a phosphoric acid system (PAFC), a melt carbonate system (MCFC), a solid oxide system (SOFC), and a solid polymer system. Among a great number of fuel cells, a direct methanol fuel cell (DMFC), which is an example of fuel cells of a solid polymer system, is the most promising energy source for use in mobile devices, electric cars, and others.
The reason for this is that the DMFC uses a methanol solution as a fuel having a theoretical energy density of 4.8 kW/l, which is ten times or more the energy density of the lithium ion secondary battery. In other words, it is possible that the fuel cell using methanol as a fuel achieves an energy density higher than that of the lithium ion secondary battery. In addition, the DMFC has advantages in that it does not need a reformer for taking hydrogen from a raw material and hence can be relatively simply designed, and that the DMFC normally operates at a low temperature of 30 to 130° C., as compared to other fuel cells.
As a fuel for the DMFC, an aqueous methanol solution is used, and the aqueous methanol solution is supplied to the anode electrode side. The aqueous methanol solution advances an oxidation reaction at the catalyst layer on the anode electrode side to form carbon dioxide. Protons generated in this instance travel through an electrolyte membrane separating the anode electrode and the cathode electrode toward the cathode electrode where the protons react with oxygen to form water. The reactions proceeding on the anode electrode side and the cathode electrode side and the DMFC as a whole are represented by the following formulae.Anode electrode: CH3OH+H2O→CO2+6e−+6H+Cathode electrode: 3/2O2+6e−+6H+→3H2ODMFC as a whole: CH3OH+3/2O2→CO2+2H2O
A theoretical voltage generated by the above reactions is 1.23 V, but an actual voltage during the power generation is about 0.6 V or less. The reason the actual voltage is lower than the theoretical voltage resides in voltage depression caused due to the internal resistances of the DMFC, and there are present internal resistances inside the DMFC, such as a resistance caused due to the reaction proceeding at each electrode, a resistance caused due to the movement of a substance, a resistance caused when protons travel through the electrolyte membrane, and a contact resistance. Energy which can be taken from methanol is represented by a product of a voltage during the power generation and an amount of electrons flowing the circuit by the redox reaction of methanol, i.e., an output current, and, the lower the voltage during the power generation, the smaller the energy which can be taken from methanol. The total amount of the electrons flowing the circuit by the redox reaction of methanol is proportional to the amount of the methanol in the fuel if the below-mentioned methanol crossover is not caused.
The DMFC has a problem of methanol crossover. The methanol crossover is a phenomenon that occurs due to both diffusion caused by the methanol concentration gradient between the anode electrode side and the cathode electrode side, and electro-osmosis such that methanol leaks from the anode electrode side to the cathode electrode side in association with water moving due to the travel of protons.
Methanol penetrates the electrolyte membrane and goes from the anode electrode side to the cathode electrode side, and the methanol undergoes an oxidation reaction at the catalyst on the cathode electrode side, so that a counter-electromotive force is generated on the cathode electrode side, thus lowering the voltage. Further, this problem leads to another one in that the methanol is not used in the power generation but consumed on the cathode electrode side.
As mentioned above, the DMFC has two problems of the lowering of the voltage due to the internal resistance and methanol crossover and the fuel consumption by methanol crossover, and these problems are factors in lowering the power generation efficiency of the DMFC.
For increasing the power generation efficiency, research and development on the improvement of the properties of materials constituting the DMFC and on the optimization of the power generation conditions of the DMFC are made.
In the research on the improvement of the properties of materials constituting the DMFC, intensive studies are conducted especially on prevention of the methanol crossover.
As a method for preventing the methanol crossover, a method in which the all methanol supplied is used up on the anode electrode side is considered. As examples of this, there can be mentioned a method in which the catalytic activity per unit amount on the anode electrode side is improved and a method in which the amount of the catalyst carried is increased. However, it is difficult to improve the catalytic activity of a Pt—Ru catalyst currently generally used in the catalyst layer for the anode electrode, and an optimal catalyst has not yet been found. The increase of the amount of the catalyst carried can improve the catalytic activity to some extent, but the increase of the catalyst causes the internal resistance to be larger, leading to a problem in that the voltage is lowered.
In addition to the research and development of new catalysts, intensive studies are also made with a view toward developing an electrolyte membrane that can minimize the methanol crossover. A polyperfluoroalkylsulfonic acid membrane {e.g., Nafion (registered trademark) membrane}, which is generally used as an electrolyte membrane for DMFC, advantageously has high Proton conductivity, but it also has high permeability to methanol. An optimal catalyst has not yet been found as mentioned above, and similarly an optimal electrolyte membrane has not yet been found.
On the other hand, an article: “Journal of Power Sources, 112, (2002) 339-352” has a description showing that the power density and power generation efficiency of a DMFC vary mainly depending on the temperature, the flow rate of the fuel supplied, and the fuel concentration, and a description about the properties of the fuel cell as the above operational environments are changed.
In this article, data of the properties of the fuel cell is collected wherein the fuel cell is operated under different operating conditions such that the temperature is 40° C., 60° C., or 80° C., the flow rate of the fuel supplied is 0.15 ml/min, 0.5 ml/min, or 5 ml/min, and the fuel concentration is 2 mol/l, 1 mol/l, or 0.5 mol/l, and it is shown that, when the fuel cell is operated at 80° C. under operating conditions such that the smallest fuel concentration is selected, i.e., 0.5 mol/l and the largest flow rate of the fuel supplied is selected, i.e., 5 ml/min, the largest power density can be obtained. The article has a description that, in the power generation by the fuel cell under the above operating conditions, the output is high, but the methanol crossover is increased, and hence the power generation efficiency becomes poor. Further, the article also has a description that a power generation efficiency required for obtaining the same output markedly varies depending on the operating conditions.
For solving this problem, a method is considered in which the properties of the fuel cell are measured while finely changing parameters, such as the temperature, the flow rate of the fuel supplied, and the fuel concentration, and the operating conditions under which a high power generation efficiency is obtained are preliminarily determined to prepare a database, and the operation of the fuel cell is controlled based on the database.
Further, as a method for optimizing the operating conditions, there has been proposed a method in which a fuel flow rate control means for controlling the flow rate of the fuel supplied from a fuel tank according to the fuel concentration is utilized to improve the performance of the fuel cell [see, for example, Patent document 1 {Japanese Patent Application Publication No. 2003-22830 (page 9, column 16, line 26 to page 11, column 20, line 35, FIGS. 8 to 12)}]. In this case, the internal model of a DMFC is represented by a mathematical formula, and the power generation conditions are grasped in the form of a formula to obtain the operating conditions.